N.B. I have updated this post on the legality of the blockade as I have continued to study the question
Leave it to the Netanyahu government (and its predecessor, under Ehud Olmert) to jump off a cliff from what should have been the moral high ground–“we withdrew from Gaza, which was then taken over by an internationally condemned terrorist group that lobs missiles into our cities indiscriminately”–into a deep, deep pit of international condemnation.
The JTA reports that the Israeli encounter with the Mavi Marmara, a Comoros flagged vessel, took place 70 miles from shore. Israel, I am sorry to say, has not signed the Convention on the Law of the Sea (the United States has signed but not ratified the treaty). But if we regard the Convention as customary international law, as I think we should, it seems clear to me that leaving aside for the moment the impact of a declared blockade, under Article 110 of the Convention, Israel would have no right to board the ship on the high seas.
It may be that Israel could justify the act as a blockade in the strict sense. There is some question (discussed in the comments below) about whether the boarding was legal because it was outside of territorial waters. Still, as various Israeli commentators recognized before the raid, the problem was really a PR problem–how to avoid being cast as the British in the story of the Exodus in reverse.
Now, there seems to be little question that the supposedly peace-loving “activists” were not exactly engaged in non-violent resistance. I have no doubt that the Israeli sailors were right to defend themselves. But still, for the Israelis to put themselves in this position–sheer incompetent awfulness.